The present invention relates to a ball screw for use in a machine tool and, especially, to a ball screw of a tube circulation type.
In a machine tool, as a mechanism for converting the rotation of a motor into a linear motion, there is used, for example, such a ball screw 10 as shown in FIGS. 21 and 22. This ball screw 10 is structured in the following manner. Namely, a screw shaft 11 includes a first spiral groove 12 formed in the outer peripheral surface thereof. In the inner peripheral surface of a nut 13 disposed on the outer periphery of the screw shaft 11, there is formed a second spiral groove 14 such that it is opposed to the first spiral groove 12. Also, in the ball screw 10, between the first and second spiral grooves 12 and 14, there are interposed a large number of balls 15 in such a manner that they are allowed to roll between them. In case where the screw shaft 11 is rotated, the balls 15 are allowed to roll between the first and second spiral grooves 12 and 14 due to a frictional force and, with the rolling motion of the balls 15, the nut 13 is allowed to move in the axial direction of the screw shaft 11.
By the way, to make the nut 13 of the ball screw 10 move continuously in the axial direction of the screw shaft 11, the balls 15 must be rolled repeatedly between the first and second spiral grooves 12 and 14. For this purpose, conventionally, there are formed, in the nut 13, ball circulation holes 16 and 17 which are used to discharge the balls 15 from a rolling passage formed between the first and second spiral grooves 12 and 14 to the outside of the nut 13 or introduce the balls 15 into the rolling passage from the outside of the nut 13, and, the two end portions of a ball circulation tube 18 are respectively inserted into these ball circulation holes 16 and 17 to thereby roll the balls 15 repeatedly. By the way, in FIGS. 21 and 22, reference character 19 designates a mounting metal member which is used to mount the ball circulation tube 18 onto the nut 13.
However, in the above-mentioned conventional ball screw, as a ball circulation tube which forms a circulation passage for the balls 15 in the outside of the nut 13, there is used such a ball circulation tube as shown in FIG. 23, that is, the ball circulation tube 18 whose section along the diameter direction thereof has a shape approximate to a circle and, therefore, as shown in FIG. 24, in the curved portions of the ball circulation tube 18, the balls 15 are arranged in a line on the outside thereof. In the case of such ball arrangement, as shown in FIG. 25, due to the mutual sliding motion between the balls which occurs within the ball circulation tube 18, the torque of the ball screw is caused vary, which has an ill influence on the operating performance of the ball screw.
The present invention aims at eliminating the drawbacks found in the conventional ball screw. Accordingly, it is an object of the invention to provide a ball screw which can prevent the torque to the ball screw from varying greatly due to the mutual sliding motion between the balls which occurs within the ball circulation tube.
The above object can be attained by a ball screw according to a first aspect of the present invention, comprising:
a screw shaft including a first spiral groove formed in the outer peripheral surface thereof;
a nut disposed on the outer periphery of the screw shaft, said nut having a second spiral groove formed in its inner peripheral surface in such a manner that it is opposed to the first spiral groove;
a number of balls rollably held by and between the second spiral groove and first spiral groove; and,
a ball circulation tube for forming a circulation passage for the balls in the outside of the nut,
wherein the ball circulation tube has such a shape that the movable amount of the balls in a first direction intersecting at right angles to the axis of the ball circulation tube is larger than the movable amount of the balls in a second direction intersecting at right angles to the first direction, and the ball circulation tube is fixed to the nut in such a manner that the outer surface of the ball circulation tube extending in parallel with the first direction is opposed to the outer surface of the nut.
Further, the above-mentioned object of a second aspect of the present invention would be invention, in a ball screw as set forth in the first aspect of the present invention, in a ball line formed by the large number of balls, there are inserted a plurality of spacers each having a modulus of longitudinal elasticity smaller than the modulus of longitudinal elasticity of each of the balls.
According to a third aspect of the invention, in a ball screw as set forth in the first or second aspect of the invention, the balls are respectively made of steel material and the spacers are respectively made of resin.
According to a fourth aspect of the invention, in a ball screw as set forth in the second or third aspect of the invention, the spacer is formed in a spherical shape almost identical in size with the ball.
According to a fifth aspect of the invention, in a ball screw as set forth in the second or third aspect of the invention, each of the spacers is formed in a disk-like shape having a diameter smaller than the diameter of each of the balls and also includes two concavely spherical surfaces respectively formed in the two end portions thereof that are opposed to the balls, while each of the concavely spherical surfaces has a radius of curvature larger than the radius of each of the balls.
According to a sixth aspect of the invention, in a ball screw as set forth in any one of the second to fifth aspects, the spacers are respectively interposed between the balls.
According to a seventh aspect of the invention, there is provided a ball screw, comprising: a screw shaft including a first spiral groove formed in the outer peripheral surface thereof; a nut disposed on the outer periphery of the screw shaft; a second spiral groove formed in the inner peripheral surface of the nut in such a manner that it is opposed to the first spiral groove: a large number of balls rollably held by and between the second spiral groove and first spiral groove; and, a ball circulation tube for forming a circulation passage for the balls in the outside of the nut, wherein, in a ball line formed by the large number of balls, there are inserted a plurality of spacers each having a modulus of longitudinal elasticity smaller than the modulus of longitudinal elasticity of each of the balls.
In a ball screw according to the first aspect of the invention, since the balls roll within the ball circulation tube in such a manner that they are arranged in a zigzag manner, variations in the torque of the ball screw caused by the mutual sliding movements of the balls occurring within the ball circulation tube can be restricted.
According to the second to seventh aspects of the invention, in case where the number of balls existing within the ball circulation tube increases and a so called ball packed phenomenon is thereby caused to occur within the ball circulation tube, the spacers are elastically deformed in the arrangement direction of the balls according to the ball packed amount (the varying amount of the number of balls existing within the ball circulation tube) and thus the ball packed amount can be absorbed by the spacers. This can restrict the variations in the torque of the ball screw which are caused by the ball packed phenomenon.
Also, according to the fourth aspect of the invention, since the balls are prevented from colliding with each other, there can be prevented generation of noises which would otherwise occur due to the mutual collision between the balls.